This invention relates generally to light extraction from LEDs, as for example to be directed into light pipes; and more specifically concerns provision of a non-imaging optical element that receives light from an LED, or array of LEDs, and efficiently redirects the light into a light pipe, without allowing any air gaps between the LED or LEDs and the light pipe entrance.
Recently, light-emitting diodes (LEDs) have become commercially important illumination sources for high-luminance applications. LEDs are miniature semiconductor light sources installed as a thin, active layer within a block of partially transparent material with high index of refraction. For example, Aluminum Indium Gallium Phosphide has optical index 3.6, while Indium Gallium Nitride has index 2.4. The high index has two deleterious effects that act to trap light within the absorbing semiconductor, especially when the die block is in air. First, total internal reflection (TIR) reduces the active layer""s nearly-Lambertian emission out the top of the die by about 1/n2, where n is the optical index value. Second, this fraction of directly emitted light is further subject to Fresnel reflectance, which varies from [(nxe2x88x921)/(n+1)]2 at normal incidence, or 25% at n=3, to unity at the TIR angle. Thus, about 90% of the emitted light suffers at least one reflection, and must propagate through the lossy semiconductor medium to another die boundary, and most likely suffer yet another reflection.
LEDs typically employ a transparent encapsulant, often of epoxy, with refractive index about 1.5. This increases the emission fraction to one fourth, and reduces Fresnel reflectance to one ninth (ironically, swapping their values). The two most commercially significant encapsulant geometries are the bullet shaped lens and the globbed die-on-board (i.e. encapsulant covering the die mounted on a circuit board). In both cases, however, light that does escape the die must undergo further light trapping of about 50%, offsetting the full improvement possible. The losses of the bullet lens are inherent in its convenient shape, presently manufactured by the billions.
It is a major object of the invention to provide a solution and/or solutions to the above problems and difficulties. Basically, a non-imaging coupler is provided, as in the form of a figure of resolution (about the optical axis), and comprises
a) a light transmitting body defining a recessed input cavity, a transparent-droplet encapsulant for a light emitting diode, or for an array of diodes in the cavity,
b) there being a body curved side wall shaped to totally internally reflect all the light emitted from the LED and encapsulant within a predetermined distance from the diode or center of said array,
c) there being a body cylindrical transition section extending from said curved side wall, and forwardly, and
d) a planar exit face at the forward end of the body, transverse to an axis of said figure of revolution.
It is another object to provide, in association with the coupler, an endwise transparent rod that is juxtaposed to said body exit face, in order to receive light from the coupler. A bend is typically provided at the distal end of the rod when delivering light to a dental-curing site in a patient""s mouth.